An automatic voltage regulator (hereinafter, also called “AVR”) is known which keeps a voltage output from a generator winding to a previously set voltage by controlling a current supplied to a field winding by a voltage generated to an excitation winding.
FIG. 10 shows a prior art of an alternating generator having an AVR. In FIG. 10, the generator 100 has a field winding 102, a generator winding 103, and an excitation winding 104. A permanent magnet 106 is disposed to a rotor 105 around which the field winding 102 is wound. A rectifier 108, a voltage detection circuit 109, and a transistor 110 are provided in the AVR 107. An output side of the rectifier 108 is connected to the field winding 102 through a brush 111, and a flywheel diode 112 and a smoothing capacitor 113 are connected in parallel with the field winding 102. The excitation winding 104 is connected to an input side of the rectifier 108. The rotor 105 is rotated by a drive source such as an engine (not shown).
In FIG. 10, when the rotor 105 is rotated by the drive source, a current is generated to the excitation winding 104 by a fluctuation of a magnetic flux of the permanent magnet 106 cutting across the excitation winding 104. The current is rectified by the rectifier 108 and supplied to the field winding 102 as a direct exciting current. The voltage detection circuit 109 compares a voltage value representing an output voltage of the generator winding 103 with a preset voltage value (target voltage) and turns on and off the transistor 110 depending on whether the output voltage is larger or smaller than the target voltage. When the transistor 110 is turned on, a current flows to the field winding 102 and an output of the generator winding 103 is increased, whereas when the transistor 110 is turned off, no current flows to the field winding 102 and the output of the generator winding 103 is decreased. An output of the generator 100 is kept constant by a repetition of the on/off operation of the transistor 110, that is, by a control of an on/off duty ratio of the transistor. An alternating generator having the AVR as described above is described in, for example, Patent Documents 1, 2, 3, and the like.